The present invention relates to a method for controlling the operation of a satellite and, more particularly, to a method for controlling the operation and consumption of fuel of a satellite near the end of its useful life.
Spacecraft propulsion systems typically use at least two tanks that store propellant therein. The propellant is used to maintain the satellite orbit due to various perturbations in space. A small amount of fuel is required to maintain an orbit. When a satellite nears the end of its fuel, the satellite may be placed in a graveyard orbit.
A standard method for determining the propellants on a spacecraft in orbit is to keep track of the propellant by bookkeeping. That is, keeping track of the burn times and thus deducing the amount of fuel used which can be subtracted from the propellant loaded prior to launch. This method is typically no more accurate than one percent of the loaded propellant. This amount may be equivalent to several months of station keeping. Because the spacecraft is deorbited several months before the end of its useful life, potentially millions of dollars in revenue are lost.
The present invention proposes a method that can be used on existing and future spacecraft to predict the amount of fuel therein without incorporating expensive fuel gauge type devices.
The present invention provides a method for determining an amount of fuel to be saved for end-of-life maneuvers so that the satellite can be controllably deorbited without shortening its regular useful life.
In one aspect of the invention, a method of operating a satellite comprises a method for operating a satellite so that a predetermined amount of fuel is left to perform end-of-life maneuvers. A first pressure differential is established between a first propellant tank and a second propellant tank. Propellant is transferred from the second tank to the first tank. The second tank is used for orbit maintenance. The first tank is used for end-of-life maneuvers. Propellant may be transferred between the tanks using a latching process so that a predetermined amount of fuel may be transferred from an empty tank to a full tank. Another way in which the propellant may be measured is to equalize the amount of propellant in each tank and using one tank to completion so the amount remaining is known.
One advantage of the invention is that the present method may be used both on existing satellites and future satellites. By utilizing the present invention the end-of-life of the satellite is known to within days rather than months. Thus, any remaining station keeping and deorbit maneuvers with the banked (transferred) propellant may be confidently performed. It should be also noted that the present invention applies to both three axis and spin stabilized spacecraft and can be applied to both mono-propellant and bi-propellant propulsion systems.
Other aspects and advantages of the present invention will become apparent upon the following detailed description and appended claims, and upon reference to the accompanying drawings.